1. Electron Sources
Researchers have been working on developing electron sources using carbon nanotubes (CNTs) for about ten years. One of the earliest references to this work is the patent of Keesmann et al. (U.S. Pat. No. 5,773,921). Some examples of the applications of using CNT electron sources are for displays (field emission displays and cathode ray tubes are two examples), e-beam lithography, x-ray sources and microwave devices (traveling wave tubes, klystrons, magnetrons, etc.). Some of these applications require high currents and high current densities, in the range of 1-100 Amps/cm2, in both pulsed and continuous wave (CW) or direct current (DC) modes. Many of these applications requiring high current densities are now being met using hot (thermal) cathodes of various types. All of these hot cathodes require power to heat the cathode and maintain its temperature in the range of 1000° C.
Other cold cathode technologies exist, but many of these require fabricating arrays of micron-size microtips. These are expensive to fabricate and not very reliable in extreme environments. This is evidenced by the fact that several companies that have made an effort to make microtip-based field emission displays have recently abandoned their efforts. Trying to incorporate microtip cathodes into microwave and x-ray devices has also met with limited success. On the other hand, carbon nanotube electron sources have been made with very inexpensive processes (such as printing or dispensing) over large areas.
Gated microtip electron sources, despite their weaknesses, did have an advantage of generating high current densities. (SRI International claimed 11.6 Amps/cm2 at 250V, “Application of Field Emitter Arrays to Microwave Power Amplifiers,” D. R. Whaley et al., Abstracts of the International Vacuum Electronics Conf, May 2-4, 2000, Monterey, Calif.; NEC Corporation claimed 1.27 Amps/cm2 from a Si microtip gated device “Field-Emitter-Array Cathode-Ray-Tube (FEA-CRT),” K. Konuma et al., SID 99 Digest p. 1151, 1999; Extreme Devices claimed 4 Amps/cm2 using what they claim as “diamond cathode technology,” Spec sheet for E-Chip ED138-250 dated March 2003—Rev. 2; see also “A Micromachined Vacuum Triode Using a Carbon Nanotube Cold Cathode,” C. Bower, et al., IEEE Trans on Electron Devices, Vol. 49, No. 8, p. 1478, August, 2002.)
The literature of carbon nanotube electron sources has examples of achievement of a few Amps/cm2. (E.g., claim of 4 Amp/cm2 with total current of only 0.4 mA in “Large current density from carbon nanotube field emitters,” W. Zhu et al., App. Phys. Let. Vol. 75, No. 6, p. 873, August, 1999.) Most of these claims were sources operated in a diode mode (ungated, anode and cathode only) and thus are of limited use for the applications of interest. What is needed is a gated electron source using carbon nanotube cathodes that can achieve high current densities. Some attempts have been made to make a gated source using carbon nanotubes (one example is D. S. Y. Hsu, et al., “Integrally Gated Carbon Nanotube-on-Post Field Emitter Arrays”, App. Phys. Lett., Vol. 80., p 118, 2002). The best that has been achieved is on the order of 0.1 Amps/cm2.
There are a couple of reasons why gated, high current density electron sources have not been made. The CNT cathodes are not regular arrays of nanotubes that are positioned in an exact formation and aligned in an exact direction. Instead, they are irregularly positioned and randomly oriented. In some cases, the alignment is preferential in a certain direction; but, unless the position and the alignment of the CNTs are engineered precisely, it will be difficult to design and engineer an optimized gated structure such as is done for microtip sources. The lack of optimization leads to poor efficiency of the emitted electrons (many of them strike the gate structure, creating heat that will ultimately lead to device destruction) and poor use of cathode area (much of the area is dedicated to gate structure and not CNT emitters). Many of the carbon nanotubes are also not optimized for high current electron emission. They can unravel or become hot and disintegrate. Increasing the density of carbon nanotubes is not a solution either because they electrically screen each other from the applied electric field needed to extract the electrons from the nanotubes (see Jean Marc Bonard et al. “Tuning the Field Emission Properties of Patterned Carbon Nanotube Films,” Advanced Materials, 13, 184 (2001)). Thus, there is a need to increase the means of increasing the current density of gated electron emission devices using CNT cathodes.
One means of increasing the current density is to use an approach that is similar to what van der Vaart et al. have described in U.S. Patent Application Publication US 2002/0053867 A1 (see also International Publication Number WO 00/79558 A1). This approach is also described in papers published in the SID literature (“Technology for the Hopping Electron Cathode,” P. J. A. Derks, et al., SID 02 Digest, p. 1396; “A Novel Cathode for the CRTs based on Hopping Electron Transport,” N. C. van der Vaart,” SID 02 Digest, p. 1392; “A Novel Electron Source for CRTs,” van der Vaart et al., Information Display, Vol. 18, No. 6, p. 14, June 2002).
In this Hopping Electron Cathode (HEC) approach, the electrons from a thermal cathode are “condensed” by a funnel-shaped structure that is coated with a layer of secondary electron emitter material. FIG. 1 illustrates how this approach works. The electrons from a hot cathode are extracted by use of a gauss electrode (grid) from the cathode and then strike the funnel. Secondary electrons are generated when the voltage of the electrode at the top of the funnel is sufficiently high enough (about 300-500V). Because the funnel surface is insulating and charge conservation must be maintained, the current is neither amplified nor degraded, but collected by the funnel to the opening at the end of the funnel. Using this approach, very high electron current densities can be emitted from the funnel opening, exceeding 1000 Amps/cm2.
2. Welding
With smaller and smaller structures and assemblies required for many applications, there is a need for assembly and welding technologies for the smaller structures. As just one example, there is a need for welding fine hydrogen separation membranes into very small reactors (micro-reactors). There is also a need for heat treatment on a fine scale and with high resolution. High throughput is also required for product manufacturing. There are several methods for welding two pieces of material together.
Contact welding (tack welding)—This involves forcing high current in a short pulse though the two parts. Typically, the joint between the two parts is highly resistive compared to the bulk of the materials and this area is heated rapidly by the pulse current. The temperature can rise to near or over the melting point of one or more of the materials and a bond is created between the materials. Typically, the size scale for this type of welding is on the order of 1 mm or larger. In this case, both parts must be metallic.
Wire bonding—Wire bonding is similar to contact welding. Ultra-sound can be applied in addition to high pulse current to create a bond. The size scale is on the order of 0.1 mm and can be highly automated. This is good for making interconnects to integrated circuits and printed circuit boards, but limited in making other assemblies.
Laser bonding—A laser can be focused to a small spot and create local heating to make a bond. Mirrors on micropositioners can direct the beam to many different spots. This approach is flexible but it is difficult to make a multibeam system to increase the throughput. In addition, metals reflect a large percentage of the light, decreasing the efficiency of the welder. The size of the spot is on the order of 0.1 mm to 0.01 mm.
Focused Ion Beam (FIB)—FIB systems are much like scanning electron microscopes (SEMs). FIBs can focus a beam to nano-scale sizes; 10 nanometer features have been demonstrated. This approach can achieve the fine resolution required for many applications, but FIB machines are expensive systems and the throughput is very low because only one beam is available to do all the machining. FIB systems are typically used for micromachining by etching material away and are not used for welding.
Electron beam welders or Scanning Electron Microscopes (SEMs)—Electron beam welders use a electron gun to weld joints in a vacuum environment Typically, the focus of the electron beam is 0.1 mm to 1.0 mm. SEMs can focus to much finer resolutions, but typically have very small currents, not sufficient for welding or bonding. Both systems use only one beam to perform all the processes. The size of the beam (welding spot) and the through put of standard electron beam welders and SEM machines are not sufficient for many nanospot welding and heat treatment applications. An e-beam welder is needed that can be sealed to an array for multibeam approaches and also achieve small beam sizes.
It is therefore a desire to provide a nanospot welder and method that addresses the need for assembly apparatus and methods for very small structures.